1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus and a semiconductor integrated circuit that turn on a discharge lamp such as a cold cathode fluorescent lamp used for, for example, a liquid-crystal display device.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a discharge lamp lighting apparatus according to a related art. In FIG. 1, bridge-connected between a DC power source Vin and a common potential (for example, the ground) are switching elements Q11 to Q14. The switching elements Q12 and Q14 are n-type MOSFETs and the switching elements Q11 and Q13 are p-type MOSFETs. Outputs from the bridge-connected switching elements Q11 to Q14 are connected through a capacitor C31 to a primary winding P1 of a transformer T1 and through a capacitor C32 to a primary winding P2 of a transformer T2.
A first end of a secondary winding S1 of the transformer T1 is connected to a first electrode of a cold cathode fluorescent lamp (hereinafter referred to as “discharge lamp”) 32. A second end of the secondary winding S1 is connected through a resistor R31 to the common potential. A second electrode of the discharge lamp 32 is connected to a first end of a secondary winding S2 of the transformer T2 and a second end of the secondary winding S2 is connected through a resistor R32 to the common potential.
An error amplifier 33 compares a voltage of a diode D31 or D33 with a reference voltage and outputs an error voltage to a PWM comparator 35. The PWM comparator 35 compares the error voltage of the error amplifier 33 with a triangular signal of a triangular wave generator 34 and generates a pulse signal whose pulse width corresponds to the error voltage. A frequency divider 36 divides the frequency of the pulse signal from the PWM comparator 35 and outputs two drive signals for every pulse to drivers 37 and 38, respectively. The driver 37 provides the switching element Q11 with the signal from the frequency divider 36 and the switching element Q12 with an inverted signal of the signal from the frequency divider 36. The driver 38 provides the switching element Q13 with the signal from the frequency divider 36 and the switching element Q14 with an inverted signal of the signal from the frequency divider 36.
As a result, a period in which the switching elements Q11 and Q14 are simultaneously ON and a period in which the switching elements Q12 and Q13 are simultaneously ON are determined according to voltages detected with the resistors R31 and R32. The switching elements Q11 and Q12 or the switching elements Q13 and Q14 never simultaneously turn on. The period in which the switching elements Q11 and Q14 are simultaneously ON and the period in which the switching elements Q12 and Q13 are simultaneously ON alternate.
Operation of the discharge lamp lighting apparatus of FIG. 1 will be explained. When the switching elements Q11 and Q14 are turned on, the DC power source Vin passes a current through a path along Q11, C31, P1, Q14, and the common potential, to apply a voltage to the capacitor C31 and primary winding P1. As a result, the capacitor C31 and an inductance of the primary winding P1 resonate to form a sinusoidal current. When the switching elements Q11 and Q14 are turned on, the DC power source Vin passes a current through a path along Q11, C32, P2, Q14, and the common potential, to apply a voltage to the capacitor C32 and primary winding P2. As a result, the capacitor C32 and an inductance of the primary winding P2 resonate to form a sinusoidal current.
The secondary windings S1 and S2 are wound to generate high voltages that are sufficient to turn on the discharge lamp 32. Namely, the secondary windings S1 and S2 generate high voltages VL1 and VL2 of sinusoidal waves with opposite phases. As a result, the secondary side passes a current through a path along S1, 32, S2, R32, R31, and S1, to turn on the discharge lamp 32. The resistor R32 generates a voltage proportional to a current passed through the discharge lamp 32. This voltage is supplied through the diode D33 to the error amplifier 33. The resistor R31 generates a voltage that reversely biases the diode D31 and turns off the diode D31, which then provides no voltage.
When the switching elements Q12 and Q13 are turned on, the DC power source Vin passes a current through a path along Q13, P1, C31, Q12, and the common potential, to reversely apply a voltage to the capacitor C31 and primary winding P1. As a result, the secondary winding S1 produces a high voltage of sinusoidal wave with an opposite phase. Also, the DC power source Vin passes a current through a path along Q13, P2, C32, Q12, and the common potential, to normally apply a voltage to the capacitor C32 and primary winding P2. As a result, the secondary winding S2 generates a high voltage of sinusoidal wave with a normal phase. The secondary side passes a current through a path along S2, 32, S1, R31, R32, and S2, to turn on the discharge lamp 32. The resistor R31 generates a voltage proportional to a current passed through the discharge lamp 32. This voltage is supplied through the diode D31 to the error amplifier 33. The resistor R32 generates a voltage that reversely biases the diode D33 and turns off the diode D33, which then provides no voltage.
Consequently, the error amplifier 33 provides a current detection signal formed by alternately combining voltages generated by the resistors R31 and R32. According to the current detection signal, the PWM comparator 35 generates a pulse signal to turn on/off the switching elements Q11 to Q14, thereby controlling a current passed to the discharge lamp 32 to a constant value. The resistors R31 and R32 detect currents passing on the low-voltage sides of the secondary windings S1 and S2 of the transformers T1 and T2 that are arranged on each side of the discharge lamp 32, and the switching elements Q11 to Q14 arranged on each side of the discharge lamp 32 are PWM-controlled with the same pulse width to generate voltages of opposite phases on each side of the discharge lamp 32.
Another related art is disclosed in Japanese Unexamined Patent Application Publication No. 2003-17287. This related art is a power source apparatus with ground fault protection function for lighting a cold cathode discharge lamp, capable of preventing a malfunction due to a leakage current. This apparatus provides a secondary winding with a center tap. Based on a fact that the potential of the center tap changes relative to a common potential if a leakage current occurs, the apparatus detects whether or not there is a leakage current, and if there is, stops an inverter.